Objective: To study marrow transplantation biology and immunology in large out-bred animals, primarily the dog. Approach: This is a continuing study of total body irradiation (TBI) and marrow transplantation, of high dose chemotherapy and marrow transplantation, of methods of support for animals that have no bone marrow function and of the immunologic problems of marrow transplantation. Specific areas of study include the immuno-reactivity of canine marrow transplant recipients, the nature of the tolerance observed in the healthy long-term canine chimeras, the effect on engraftment of preceding exposure to transplantation antigens by blood transfusion, the prevention and treatment of graft-vs-host-disease (GVHD), the problems of histocompatibility typing, and studies of the immunologic problems arising from long-term support by platelet and granulocyte transfusions.